1. Field of the Invention
The present invention relates to a current perpendicular to the planes (CPP) sensor wherein a current flowing through a conductive bias layer next to a free layer in the sensor stabilizes the free layer by a current field acting on the free layer.
2. Description of the Related Art
The heart of a computer is typically a magnetic disk drive which includes a rotating magnetic disk, a slider that has write and read heads, a suspension arm above the rotating disk and an actuator arm. The suspension arm biases the slider into contact with a parking ramp or the surface of the disk when the disk is not rotating but, when the disk rotates, air is swirled by the rotating disk adjacent an air bearing surface (ABS) of the slider causing the slider to ride on an air bearing a slight distance from the surface of the rotating disk. When the slider rides on the air bearing the actuator arm swings the suspension arm to place the write and read heads over selected circular tracks on the rotating disk where field signals are written and read by the write and read heads. The write and read heads are connected to processing circuitry that operates according to a computer program to implement the writing and reading functions.
An exemplary high performance read head employs a current perpendicular to the planes (CPP) sensor for sensing the magnetic field signals from the rotating magnetic disk. The sensor includes a nonmagnetic electrically conductive or electrically nonconductive material spacer layer sandwiched between a ferromagnetic pinned layer and a ferromagnetic free layer. An antiferromagnetic pinning layer typically interfaces the pinned layer for pinning the magnetic moment of the pinned layer 90xc2x0 to an air bearing surface (ABS) wherein the ABS is an exposed surface of the sensor that faces the rotating disk. The sensor is located between ferromagnetic first and second shield layers. First and second leads are connected to a bottom and a top respectively of the sensor for conducting a current perpendicular to the major thin film planes (CPP) of the sensor. This is in contrast to a CIP sensor where the current is conducted in plane parallel to the major thin film planes (CIP) of the sensor. A magnetic moment of the free layer is free to rotate upwardly and downwardly with respect to the ABS from a quiescent or zero bias point position in response to positive and negative magnetic field signals from the rotating magnetic disk. The quiescent position of the magnetic moment of the free layer, which is parallel to the ABS, is when the current is conducted through the sensor without magnetic field signals from the rotating magnetic disk.
When the aforementioned material spacer layer is nonmagnetic and electrically conductive, such as copper, the current is referred to as a sense current, but when the material spacer layer is nonmagnetic and electrically nonconductive, such as aluminum oxide, the current is referred to as a tunneling current. Hereinafter, the current is referred to as a current (I) which can be either a sense current or a tunneling current.
When the magnetic moments of the pinned and free layers are parallel with respect to one another the resistance of the sensor to the current (I) is at a minimum and when their magnetic moments are antiparallel the resistance of the sensor to the current (I) is at a maximum. Changes in resistance of the sensor is a function of cos xcex8, where xcex8 is the angle between the magnetic moments of the pinned and free layers. When the current (I) is conducted through the sensor, resistance changes, due to field signals from the rotating magnetic disk, cause potential changes that are detected and processed as playback signals. The sensitivity of the sensor is quantified as magnetoresistive coefficient dr/R where dr is the change in resistance of the sensor from minimum resistance (magnetic moments of free and pinned layers parallel) to maximum resistance (magnetic moments of the free and pinned layers antiparallel) and R is the resistance of the sensor at minimum resistance.
Sensors are classified as a bottom sensor or a top sensor depending upon whether the pinned layer is located near the bottom of the sensor close to the first read gap layer or near the top of the sensor close to the second read gap layer. Sensors are further classified as simple pinned or antiparallel (AP) pinned depending upon whether the pinned layer structure is one or more ferromagnetic layers with a unidirectional magnetic moment or a pair of ferromagnetic AP layers that are separated by a coupling layer with magnetic moments of the ferromagnetic AP layers being antiparallel. Sensors are still further classified as single or dual wherein a single sensor employs only one pinned layer and a dual sensor employs two pinned layers with the free layer structure located therebetween.
The first and second shield layers may engage the bottom and the top respectively of the CPP sensor so that the first and second shield layers serve as the aforementioned leads for conducting the current through the sensor perpendicular to the major planes of the layers of the sensor. The read gap is the length of the sensor between the first and second shield layers. It should be understood that the thinner the gap length the higher the linear read bit density of the read head. This means that more bits can be read per inch along the track of a rotating magnetic disk which enables an increase in the storage capacity of the magnetic disk drive.
It is important that the free layer be longitudinally biased parallel to the ABS and parallel to the major planes of the thin film layers of the sensor in order to magnetically stabilize the free layer. This is typically accomplished by first and second hard bias magnetic layers which abut first and second side surfaces of the spin valve sensor. Unfortunately, the magnetic field through the free layer between the first and second side surfaces is not uniform since a portion of the magnetization is lost in a central region of the free layer to the shield layers. This is especially troublesome when the track width of the sensor is sub-micron. End portions of the free layer abutting the hard bias layers are over-biased and become very stiff in their response to field signals from the rotating magnetic disk. The stiffened end portions can take up a large portion of the total length of a sub-micron sensor and can significantly reduce the amplitude of the sensor. It should be understood that a narrow track width is important for promoting the track width density of the read head. The more narrow the track width the greater the number of tracks that can be read per linear inch along a radius of the rotating magnetic disk. This further enables an increase in the magnetic storage capacity of the disk drive.
There is a strong-felt need to improve the biasing of the free layer and increase the magnetic storage of a disk drive.
The invention provides a CPP sensor which has a plurality of layers wherein the layers have major planes that are perpendicular to a head surface, such as the aforementioned ABS, and layers have a stripe height that extends normal to the head surface into the magnetic head assembly. The first lead is electrically connected to the sensor for conducting a current to the sensor perpendicular to the major planes of the layers and parallel to the head surface. The second lead extends from the stripe height into the head assembly. A nonmagnetic electrically conductive bias layer, which has major planes that are parallel to the major planes of the sensor, is electrically connected to the sensor and the second lead and is sized in width so that when a current flows through the bias layer parallel to the major planes of the bias layer and perpendicular to the head surface a free layer in the CPP sensor is properly biased. The CPP sensor and the bias layer are located between first and second shield layers with the bias layer being electrically insulated from the shield layers.
The bias layer preferably interfaces the free layer of the CPP sensor so that strong flux closure is implemented between the free layer and the bias layer. One of the shield layers, such as the first shield layer, is electrically connected to the CPP sensor and serves as a first lead for the current. Accordingly, the current can flow from the first shield into the CPP sensor perpendicular to the major thin film planes of the CPP sensor to the conductive bias layer, thence parallel to the major thin film planes of the conductive bias layer to the second lead which extends from the stripe height into the head assembly. The first and second leads are connected to processing circuitry which generates the current and detects potential changes upon the occurrence of field signals, which field signals constitute playback signals in the magnetic disk drive. The CPP sensor can be either a magnetic tunnel junction (MTJ) sensor or a CPP spin valve sensor. The conductive bias layer promotes a more uniform longitudinal magnetic field through the free layer which overcomes the aforementioned problems associated with the prior art hard bias layers at each side edge of the CPP sensor.
A preferred embodiment of the invention employs first and second CPP sensors where each CPP sensor has a ferromagnetic free layer with a magnetic moment which is oriented substantially parallel to the head surface and which is free to rotate in response to a signal field from a magnetic medium. Each free layer has major thin film planes that are perpendicular to the head surface and a stripe height that extends normal to the head surface into the head assembly. A nonmagnetic electrically conductive bias layer is located between the first and second CPP sensors and has major thin film planes that are parallel to the major thin film planes of the free layers of the CPP sensors and the bias layers extend in a direction away from the head surface into the head assembly beyond the stripe heights. The first and second CPP sensors and the bias layer are located between the first and second shield layers. The first and second shield layers are electrically connected to the first and second CPP sensors respectively for conducting first and second currents respectively through the sensor perpendicular to the major thin film planes of the free layers to the conductive bias layer. The conductive bias layer is electrically connected to the first and second CPP sensors and are sized so that the first and second currents can flow through the bias layer perpendicular to the head surface to cause a biasing field which biases the magnetic moments of the free layers antiparallel with respect to one another. This results in flux closure between the free layers of the first and second CPP sensors which promotes a very stable biasing condition. Again, the conductive bias layer preferably interfaces the free layers of the first and second CPP sensors.
In both embodiments the conductive bias layer may have a width at the head surface that is substantially equal to the width of the layers of the sensor at the head surface. Optionally, the conductive bias layer may have a width at the head surface that is greater than the width of the layers of the sensor at the head surface so that the strength of the biasing of the free layer or free layers is reduced. Accordingly, the width of the conductive bias layer may be sized so as to achieve the desired biasing of the free layer or the free layers.